BTU to kJ:
multiply by 1.055055853

How many kJ are in a BTU?

One BTU is 1.055055853 kilojoules. The BTU used here is the International Table (IT) British thermal unit, the version most appliance labels and engineering tables refer to, and the site converter uses that value.

A BTU is a small amount of energy: roughly the heat needed to raise one pound of water by one degree Fahrenheit. A kilojoule is 1,000 joules, so one BTU is a little over one kilojoule. That near-equality gives a handy rule of thumb: the kJ figure is always about 5 percent larger than the BTU figure.

The BTU to kJ formula

For a quick estimate, add 5 percent to the BTU figure. For 12,000 BTU, 5 percent is 600, giving about 12,600 kJ, which is close to the exact answer.

BTU to kJ conversion chart

Use this chart for common values. Every figure comes from multiplying by 1.055055853.

Where BTU to kJ conversions come up

Gas boilers, space heaters and barbecue burners are often rated in BTU, while energy bills and scientific data sheets use kilojoules or megajoules. If a furnace burns 50,000 BTU of gas, it releases 50,000 × 1.055055853 = 52,752.79 kJ, or about 52.75 MJ.

The reverse direction matters on food and fuel labels. A 500 kJ snack bar is 500 × 0.9478171203 = 473.91 BTU of food energy. A figure like 3,600 kJ equals 3,412.14 BTU, which is exactly one kilowatt-hour expressed in BTU.

A second worked example and a sanity check

Cooking gives a smaller-scale example. A gas hob burner rated at 9,000 BTU releases 9,000 × 1.055055853 = 9,495.50 kJ of energy when it burns for one hour at full output. Recipe and stove data written in kilojoules then compare directly with the burner label.

Going the other way, a food or fuel figure of 2,000 kJ is 2,000 × 0.9478171203 = 1,895.63 BTU. Bulk gas is often priced by the therm, which is 100,000 BTU, and 100,000 BTU converts to 105,505.59 kJ, or about 105.5 MJ.

A sanity check that needs no calculator: kJ is about 5 percent bigger than BTU, so a result below the BTU figure means you divided instead of multiplying. Doubling the BTU value should double the kJ value, so 250 BTU gives 263.76 kJ, exactly one quarter of the 1,055.06 kJ you get from 1,000 BTU.

A quick size check for your answer

One BTU is a little more than one kilojoule (1.055 kJ), so the kilojoule figure is always slightly larger than the BTU figure. For example, 500 BTU is about 527.53 kJ and 5,000 BTU is about 5,275.28 kJ. If your answer is smaller than the number you started with, you divided instead of multiplied.

Food labels use kilojoules and kilocalories rather than BTU, so this conversion mostly comes up with heating and cooling equipment, gas appliances and older manuals that quote energy in BTU while newer specifications use SI units.

Common mistakes when converting BTU to kJ

• Mixing up BTU and BTU/h. One is energy, the other is power. Converting the number gives kJ or kJ/h, so keep the time part.
• Using a rounded factor of 1 for quick work on large values. At 100,000 BTU the error is over 5,000 kJ.
• Confusing BTU with MBTU or MMBTU, which usually mean thousands or millions of BTU in the fuel trade. Check the label before you multiply.
• Dividing when you should multiply. A BTU is the smaller unit, so the kJ number is always a bit higher.